It is generally known to equip an air jet loom with first and second light beam sensor devices at the outlet end of the weft insertion channel of the weaving reed, for monitoring the weft insertion and detecting any arising weft insertion faults. The second light beam sensor device is arranged slightly downstream or outwardly beside the first light beam sensor device. Thus, a properly inserted weft thread reaches and triggers the first light beam sensor device, but does not reach and trigger the second light beam sensor device. The sensor device emits a corresponding signal indicating that the weft thread has been properly inserted into the weft thread insertion channel of the weaving reed. However, in the event of a too-long weft thread or in the event of a weft break, the end of the weft thread will also reach the second light beam sensor device, such that the second sensor emits a corresponding electrical signal that will be evaluated in an evaluation unit to determine that a weft fault has occurred.
In some circumstances, one or both of the light beam sensor devices may fail to provide the proper weft detection signal, or may release an erroneous interference signal. As a result, a weft fault may fail to be detected, i.e. a signal apparently indicating a proper weft insertion may exist even though a weft fault has actually taken place. For example, during the weaving operation, the first light beam sensor device may release an interference signal that appears to be a weft detection signal. Such an interference signal may, for example, be caused by a break in the electrical cables connected to the light beam sensor device, such as the electrical cable providing power to the light beam source or the conductor carrying the detection signals. Such an interference signal may even arise during a phase of the weaving operation, i.e. in an angular position range of the main shaft of the loom, not corresponding to the angular range in the weft thread would be expected to reach the first light beam sensor device. The second light beam sensor device may also emit such an interference signal, which would appear to indicate that a weft fault has occurred, even if no weft fault has actually occurred.
On the other hand, a breakdown or failure of one or both light beam sensor devices, for example due to a cable break, may occur and go undetected. In this case, the evaluating unit may not receive signals indicating a weft fault that has actually occurred, or may receive erroneous signals apparently indicating a weft fault when a weft fault has not actually occurred. In conventional air jet looms using a stop motion including first and second light beam sensor devices located at the outlet end of the weft insertion channel, to the present day, there has been no reliable and unambiguous way to monitor the proper functionality or operability of the light beam sensor devices.
This problem exists especially with regard to the second sensor device. The first sensor device is expected to emit a weft detection signal for each properly completed weft insertion cycle. Thus, the operability of the first sensor device can be confirmed in each cycle. On the other hand, the second sensor device does not emit a weft detection signal in the normal operating condition with proper weft insertion cycles. The second sensor device only becomes active, i.e. emits a signal, rather infrequently and intermittently when a rare weft break takes place. Therefore, an inoperable second sensor may have the same signal output (e.g. no signal) as an operable second sensor indicating proper weft insertions. For this reason, it is generally not possible to detect a defect or failure of the operation of the second sensor device during each weft insertion cycle, and a failure of the second sensor device can go undetected for long periods of time, so that faulty weft threads may be woven into the cloth being produced.
Swiss Patent 655,745 discloses a method for detecting a weft insertion fault by using a weft detector arrangement provided on the loom. Such a weft detector arrangement is generally known as a stop motion, and this term will be used in the following discussion. For each rotation of the loom main shaft, the method according to Swiss Patent 655,745 defines a first rotational angular range during which a weft thread is expected to arrive at the stop motion sensors, and a second rotational angular range outside of the first range, i.e. during which a weft thread is not expected to arrive at the stop motion sensors. The stop motion arrangement monitors the presence of the weft thread during the above defined first and second rotational angular ranges. In this context, a signal indicating the presence of a properly inserted weft thread is only generated during the first rotational angular range, while a fault signal, which is not generated by a weft thread, is acquired in the second rotational angular range, and thereupon a signal which stops the loom operation is generated. The signal generated during the second rotational angular range is only evaluated as a fault signal after this signal arises continuously over a predetermined number of rotations of the loom main shaft, for example.
U.S. Pat. No. 4,487,235 (Sugita et al.) discloses a method and an apparatus for detecting a weft thread that has been inserted into the loom shed of a shuttleless jet loom, namely an air jet loom or a water jet loom. The method involves calculating a discriminating function for discriminating between the presence of the weft yarn and the presence of the jet of fluid under normal weft insertion conditions, as indicated by the signal produced by two weft sensors of a stop motion arrangement. A signal produced during actual weft detection operation is compared with the discriminating function by a statistical method to determine whether or not the weft yarn is present. The discriminating function may be based on or take into account the original wave-form of the signal, a differentiated value thereof, an integrated value thereof, frequencies in certain frequency ranges thereof, and sampling averages of the above parameters. The disclosed method and apparatus does not monitor the function of the individual weft thread sensors within the stop motion arrangement, and particularly does not monitor the function of the second or downstream weft sensor.
Published European Patent Application 0,004,836 discloses an apparatus for monitoring a weft thread using weft sensors or weft feelers in a fluid jet loom. The apparatus includes a generally conventional stop motion arrangement having first and second electrical weft sensors, which together sense and indicate a normally inserted weft thread, and having a further third weft sensor which detects a longer or broken weft thread. The apparatus and method do not provide for monitoring the functionality or operability of the individual sensors.